1. Technical Field
The present invention relates to a method and an apparatus for leveling an elevator car at a landing sill.
2. Background Art
To stop an elevator smoothly and level with a sill, an elevator system must know when to initiate a stop, when to go into a leveling mode of operation, and when to begin opening the landing doors. The elevator system initiates a leveling mode of operation when the car reaches a leveling zone in the hoistway. Most elevators begin opening the landing doors two to three inches before the elevator platform is actually level with the sill to speed-up passenger transfer. This zone is known as a door zone. To perform these functions accurately, it is necessary to know the exact location of the car at all times. Elevator position transducer devices are used to monitor car position.
One existing elevator position transducer device is a Primary Position Transducer. The Primary Position Transducer ("PPT") is a digital encoder that is located in the machine room over the hoistway. It is driven by a steel tooth-tape that is attached to and runs with the car down the length of the hoistway. An idler sheave is disposed in the hoistway pit to prevent the tape from fluttering in the hoistway. This device requires contact with the elevator and is subject to errors resulting from floor location drift. Floor drift is caused by many factors such as building settlement, building expansion and contraction, and cable expansion and contraction.
Another device for determining elevator position includes steel bars or vanes attached to a floating steel tape, running the length of the hoistway, and a hoistway position reader box mounted on the car which are used to monitor the car position. The vanes must be precisely located with respect to their corresponding landing sills to mark the approximate stopping distance from landing, the leveling zone, and the door zone. The reader box contains switches that sense the location of each vane as the car travels up and down the hoistway. This device also requires contact with the elevator and is subject to errors caused by floor drift.
Another device for elevator position detection includes a plurality magnets precisely placed at each landing sill to mark the approximate stopping distance from landing, the leveling zone, and the door zone. A sensor responsive to the magnets is attached to the elevator car. This device requires that each magnet must be precisely placed at each landing sill.
Other devices, such as governor shaft encoders or motor shaft encoders, are subject to cumulative errors from sources such as cable stretch and slippage. Still other devices, such as a electromagnetic wave phase measurement device, can measure the car location but not with precise relation to the landing sill because the sill location is subject to change as a result of building settlement, building expansion and contraction, and cable expansion and contraction.